A transaxle and an engine for an automotive vehicle usually are assembled in a common powertrain package with the axis of the engine crankshaft parallel to the transaxle output shaft axis. The crankshaft axis and the transaxle output shaft axis are transversely mounted in the vehicle in a forward powertrain compartment. The output shaft drives the steerable front wheels of the vehicle through universal joints and half shaft assemblies. A typical example of a driveline of this kind may be seen by referring to U.S. Pat. No. 4,509,389, which is assigned to the assignee of this invention.
Other examples of transaxles of this kind may be seen by referring to U.S. Pat. No. 4,607,541 and to my U.S. Pat. No. 4,418,585. Each of these prior art patents includes a pair of simple planetary gear units arranged to define a compound torque flow path with selectively engageable clutches and brakes for establishing the driving speed ratios. The planetary gearing of U.S. Pat. Nos. 4,418,585 and 4,509,389, like the planetary gear arrangement of the present invention, is disposed with its axis common to the output shaft axis. The multiple ratio planetary gear units of reference U.S. Pat. No. 4,607,541, on the other hand, are disposed on an axis parallel to the output shaft axis. That parallel axis is coincident with the hydrokinetic torque converter axis situated between the torque input elements of the gearing and the engine.
Each of these prior art reference patents describes a transaxle capable of achieving four forward driving speed ratios and a single reverse speed ratio. Each of them also describes a hydrokinetic torque converter. Each torque converter has a lockup clutch for effecting a mechanical driving connection between the impeller and the turbine of the converter.
Because of the constraints that are imposed on an engine powered vehicle driveline by the limited number of ratios available in the planetary gearing, it is not possible with such four speed ratio transaxles to achieve the full performance potential and the full potential for economy gains that would be available if five or more ratios in the driving range were to be available.
I am aware of recent attempts to modify existing four speed ratio transmissions to achieve five driving ratios in an automotive vehicle driveline, but these attempts usually involve the use of auxiliary gearing at the torque output portion of the basic four speed ratio gearing. They also include separate clutches and brakes for controlling the driving ratio of the auxiliary transmission. An example of a driveline of this kind may be seen by referring to Ward's Engine Update, an automotive news magazine, dated July 15, 1989, Page 6.
Such modifications to normal four speed ratio gear systems have only limited potential for improving efficiency and performance because of the limitations on the ratio spread available by the use of the auxiliary planetary gearing. Those limitations are imposed by the geometry of the auxiliary planetary gearing itself since the change in ratio range that is available by reason of the addition of the auxiliary gear unit is limited by the available range of pitch diameter ratios of the internal gear and the external gear of the auxiliary planetary gear system. Further, the use of such an auxiliary gear system adds to the overall length of the transmission, which makes it difficult to assemble the transmission in a compact engine and transmission compartment. In most vehicle applications, especially front wheel drive vehicles, styling considerations impose limitations on available space for the powertrain package.
Aside from such space limitations, the use of the auxiliary gear unit adds considerable weight to the transmission and adds to the complexity in manufacture and assembly as well as cost because of the additional elements that are required.